1. Field of the Invention
The present invention relates to a laser oscillator. Particularly, the present invention relates to a laser oscillator using an optical fiber which is excited by light as a gain medium.
2. Related Art
Generally, an optical fiber laser oscillator using an optical amplification fiber which is excited by light as a gain medium has been known as, for example, in Japanese Patent Application Publication No. 9-64440.
FIG. 11 shows a configuration of a conventional optical fiber laser oscillator 700. The optical fiber laser oscillator 700 includes: a optical circulator 710 in which light incident on a first port 711 is exited from a second port 712, and light incident on the second port 712 is exited from a third port 713; an EDF 720 being an optical fiber containing erbium (Er); a fiber Bragg grating (FBG) 730 that reflects light with a predetermined wavelength among the light exited from the second port 712 and injects the same into the second port 712 again, a pump light source 740 that generates an exciting light to excite the EDF 720 and an optical coupler 750.
The EDF 720 amplifies the light exited from the third port of the optical circulator 710 and injects the same into the first port 711. The optical coupler 750 is inserted into a path between the third port 713 and the first port 711 of the optical circulator 710 and injects the exciting light outputted form the pump light source 740 to the EDF 720.
In such conventional light fiber laser oscillator 700, a loop optical path formed by the optical circulator 710, the EDF 720, the FBG 730 and the optical coupler 750 includes a resonator for a laser oscillation. Thereby the optical fiber laser oscillator 700 can output the light oscillated by the laser from the FBG 730.
Meanwhile, when the laser according to the present invention is used for a light source for optical communication, it is required that the laser is oscillated in a single mode and the mode is not further hopped. However, the laser oscillation mode of the optical fiber laser oscillator 700 is easily changed, i.e. so-called mode hopping is easily generated because of any effect of temperature change. In the case of a ring fiber laser, the laser oscillated in a single mode oscillates in a mode (m) in which a gain in the resonator is maximized among modes which satisfy the following formula 11.f=m·FSR  11, where m is integer,FSR (Free Spectrum Range: longitudinal mode interval)=c/Nl, where c is light velocity, n is refractive index of optical fiber and L is cavity length
The oscillation frequency of the laser is changed dependent on the temperature change δT, and the amount of change δf is obtained by the following formula 12.δf=(α+β)δT×f  12,where α is linear thermal expansion coefficient and β is the temperature dependency of the refractive index
The oscillation frequency is changed dependent on the temperature change according to the formula 12. Then, when the oscillation frequency is arrived at a certain position by the FSR, the mode in which the gain is maximized is changed, so that the mode is hopped.
Here, if the cavity length of the laser is increased, the longitudinal mode interval FSR is reduced. Then, if the longitudinal mode interval FSR is reduced, the oscillation frequency of the laser could be changed by the FSR even if the temperature is changed a little, i.e. the mode is hopped. In the conventional optical fiber laser oscillator 700, the cavity length is several meters, so that the mode is hopped even if the temperature is changed by about 0.01 degree centigrade and is it not stable when the temperature is changed.
Thus, the advantage of the present invention is to provide a laser oscillator which is capable of solving the problem accompanying the conventional art. The above and other advantages can be achieved by combining the features recited in independent claims. Then, dependent claims define further effective specific example of the present invention.